1. Field of the Invention
The present invention relates to a microwave filter using a dielectric block and, more particularly, to a microwave filter, having U-type resonators which passes a desired frequency signal and removes an undesired frequency signal upon being used in a high frequency antenna circuit of radio communication systems such as a mobile communication, a personal communication, a satellite communication, and IMT-2000.
2. Description of the Prior Art
Recently, in a high frequency filter used in the antenna terminal in radio communication systems, dielectric block coaxial resonators of TEM mode have been widely used. The number of coaxial resonators used is determined depending on the required characteristics of a filter, generally, the filter is fabricated by means of two or more resonators.
A conventional high frequency filter structure using a dielectric block is shown in FIGS. 6 and 7, wherein the conventional high frequency filter is an integrated structure upon forming two or more resonators to a dielectric block, it is a filter structure using three resonators therein. In FIGS. 6 and 7, the dielectric block 1 is a structure in which all portions are coated with a conductive metal except a top surface 1a and coupling quantity control holes 3 which controls the coupling quantity between resonators.
The dielectric block 1 includes apertures corresponding to the resonators 2 and the apertures extending from one surface, that is, a top surface, to an opposite (bottom) surface, are arranged in a line in parallel with each other. All the surfaces of the dielectric block 1, except the top surface thereof, and inner surfaces of the coupling quantity control holes 3 are coated with a conductive film. Then, the bottom surface of the dielectric block 1 acts as a shorted portion connected to a ground voltage level and the aperture acts as resonator of the 1/4 wavelength. Also, conductive rods 5 for input and output terminals are inserted into the apertures of the first and last resonators, respectively, and dielectric materials 4 are inserted between the conductive rods and resonators for coupling between input and output terminals and resonators. Further, for properly controlling the coupling quantities, there is structured a filter whose holes 3 for controlling coupling quantities are formed between resonators respectively. The input signal through input apertures is transferred to the resonators by means of an electric field couple between the inner surface of the apertures for the input terminal arid the resonator, the signal is transferred from a front resonator to a back resonator with the electromagnetic field coupling between the resonators. Further, since the signal is transferred to the output apertures by means of the electric field coupling between the resonator and apertures for the output terminals, the energy is transferred to the output through the resonators from the input. At this time, in order to control the coupling quantity of the electric field between the resonators, there are formed holes 3 for controlling the coupling quantity. Since this coupling structure regards an inner of the hole 3 for controlling the coupling quantity as a space, it decreases the coupling quantity due to the difference of the dielectric susceptivity of the dielectric block. Accordingly, it is possible to miniaturize the filter, and the coupling quantity between the resonators can be regulated by changing the size of the hole for controlling the coupling quantity. Furthermore, the middle portion of the dielectric block 1 is strong in the electric field, the portion near to the front and back surface of the dielectric block is gradually weak in the electric field. Accordingly, since the hole position for controlling the coupling quantity is moved from the middle direction to the front or rear surface direction of the dielectric block, the coupling quantity is regulated. However, it is difficult to accomplish a miniaturization since the control of the coupling quantity by means of the position movement or the size of a hole for controlling the coupling quantity has a limit according to the miniaturization of parts. Moreover, the inner surface of the resonators is coated with a conductive metal and the inner surface of the hole 3 for controlling the coupling quantity is not coated with a conductive metal. Therefore, when the inner surface of the resonators is coated with a conductive metal after fabricating the dielectric block 1, it is problematic that the fabrication process is complex since the following process is required in order that the inner surface of the hole for controlling the coupling quantity is not coated with a conductive metal, the undesired signal is transferred through the opened surface of the dielectric block by means of the transferred signal to the input side. In view of the frequency characteristics of these wide band pass filters, the attenuation characteristics of the frequency higher than the pass wide band is equal to that of the frequency lower than it as FIG. 8. However, for an efficient usage of frequency in the radio communication system, since the transmitting and receiving frequency wide band are positioned near to each other, for the high frequency filter, there is required an excellent attenuation characteristics in the near band to the stop band.
To improve the attenuation characteristics in the stop band, if there are used a lot of numbers of resonators in the filter, the attenuation characteristics are improved but the insertion loss increases and the filter size increases. Accordingly, a filter having a pole, which improves the attenuation characteristics upon cutting off signals at a specified frequency with a chip capacitor or inductor in the external without increasing the number of resonator, has been designed. However, there is a problem that the fabrication process is complex because of using the external chip device.
In the filter structure of FIG. 7, as another conventional art, in order to control a coupling quantity between the resonators, grooves 6 are formed for controlling the coupling quantity from the front and rear surface of a dielectric block 1 to the top and bottom surface of the dielectric block, coated with the conductive metal. Such a structure can simplify a fabrication process because of decreasing the following process so that the inner surface of the coupler may not be the conductive metal. However, since the middle portion of the dielectric block is strong in the electric field and the front or rear surface of the dielectric is weak in the electric field, the control of the coupling quantity using the grooves for controlling this coupling quantity is very weak, there is a limit for the miniaturization of parts. That is, there has been still a problem which improves the attenuation characteristics in the stop band.